In this paper we have analysed the Common Mode Rejection Ratio (CMRR) for differential amplifiers used in bioimpedance measurement systems and derived the complete equations for the case when OPAMPs have finite differential and common mode gains. In principle, passive ac-coupling networks that include no grounded components have an infinite CMRR, but they must provide a path for input bias currents. The paper provides a novel approach as to how component tolerances limit the CMRR and affect the transient response of different networks. Experimental results and various measurements support our theoretical predictions. The best CMRR is obtained when the differential gain is concentrated in the input stage, but it decreases at frequencies above 1 kHz because of the reduced CMRR for the differential stage at these frequencies.